A driving circuit integrated type active-matrix liquid crystal display device includes a plurality of display pixel sections and pixel use thin film transistors for controlling these display pixel sections on a substrate. The driving circuit integrated type active-matrix liquid crystal display device further includes a video signal driving circuit and a scanning signal driving circuit formed on the substrate for driving the pixel use transistors by the dot sequential method.
The described conventional driving circuit integrated type active-matrix liquid crystal display device will be explained with reference to FIG. 2 which explains the present invention. The liquid crystal display device is arranged such that a video signal line 7 from a video signal driving circuit 1 i s connected to a source of a pixel use thin film transistor 4 for controlling a display pixel section 3 composed of a liquid crystal layer, while connected to the gate of the pixel use thin film transistor 4; is a scanning signal line 6 from the scanning signal driving circuit 2. To a drain of the pixel use thin film transistor 4 are connected the display pixel section 3 and an auxiliary capacitance 5.
The video signal driving circuit 1 includes shift registers 9, buffers 10, and an analog switch 11. The scanning signal driving circuit 2 includes shift registers 9 and buffers 10.
For a conventional buffer to be deposited on the video signal driving circuit 1 or the scanning signal driving circuit 2, for example, a buffer 62 as illustrated in FIG. 13 composed of a P-channel thin film transistor 60 and an N-channel thin film transistor 61 which are connected in series between a high potential power source Vdd and a low potential power source Vss may be adopted.
However, the liquid crystal display device provided with such buffer 62 has the following drawbacks. That is, a line-shaped defect due to damage on the thin film transistors 60 and 61 which constitute the buffer 62 is likely to generated, and an improved yield of panels are difficult to be achieved.
Specifically, in the buffer 62 provided in the scanning signal driving circuit 2, if a defect occurs in either one of the transistors 60 and 61, a normal signal would not be inputted to the pixel thin film transistor 4 connected to the defective pixel thin film transistor via the scanning signal line 6. This causes the problem that the display pixel section 3 on the scanning signal line 6 is not lightened properly, thereby generating a line-shaped defect.
The possible causes of such defect of the thin film transistors 60 and 61 which constitute the buffer 62 may be dust, static-electricity, etc. In order to prevent an electrostatic breakdown (breakdown due to the static-electricity) of the thin film transistor, Japanese Laid-Open Patent Application No. 260650/1994 (Tokukaihei 6-260650) discloses a technique of raising a withstanding voltage of the entire thin film transistor by forming an end portion of the thin film semiconductor region that shows a relatively weak resistance to static-electricity by an intrinsic semiconductor or the same conductor as a channel forming region so as to raise the withstanding voltage. Although the described method permits a generation of the defective thin film transistor to be suppressed, it fails to provide a solution to completely prevent a generation of defect.
On the other hand, Japanese Laid-Open Patent Application No. 123896/1994 (Tokukaihei 6-123896) discloses the arrangement where a plurality of thin film transistors which constitute the buffer are connected in parallel in order to achieve improved characteristics of the thin film transistor by the hydrogenating process. Video signal writing switch thin film transistors are shown in FIG. 14 as an example of the arrangement where the thin film transistors are connected in parallel. The video signal wiring switch thin film transistor shown in FIG. 14 includes a thin film transistor 63, a video signal line input 64 and a shift register output line 65, for inputting a shift register output as a gate signal, and a video signal line output 66.
However, with such simple parallel connection, when separating the defective thin film transistor 63 using a laser beam, etc., it is required to cut off the video signal line input 64 or a semiconductor layer in a narrow space between the video signal line output 66 and the shift resistor output line 65. Therefore, it is likely that the shift register output line 65 maybe damaged when cutting off the semiconductor layer, and an improved yield of panels is difficult to be achieved by modifying the buffer.
In general, a buffer has a large sized transistor, which requires a long wire for the gate line, thereby presenting another problem that an electrostatic breakdown of the thin film transistor due to the antenna effect is likely to occur.